foo.go

package main

import (
	"crypto/rand"
	"crypto/rsa"
	"crypto/x509"
	"encoding/pem"
	"fmt"
	"log"
	"os"
)

func main() {
	// Read the entire file containing the PKCS#8-encoded private key
	pemData, err := os.ReadFile("rsakey.pem")
	if err != nil {
		log.Fatalf("Failed to read file: %v", err)
	}

	// Decode the PEM block
	block, _ := pem.Decode(pemData)
	if block == nil {
		log.Fatal("Failed to decode PEM block containing private key")
	}

	// Ensure the block type is "PRIVATE KEY" (PKCS#8)
	if block.Type != "PRIVATE KEY" {
		log.Fatalf("Expected a block of type 'PRIVATE KEY', but got '%s'", block.Type)
	}

	// Parse the PKCS#8-encoded private key
	parsedKey, err := x509.ParsePKCS8PrivateKey(block.Bytes)
	if err != nil {
		log.Fatalf("Unable to parse PKCS#8 private key: %v", err)
	}

	// Type-assert the private key to *rsa.PrivateKey
	privateKey, ok := parsedKey.(*rsa.PrivateKey)
	if !ok {
		log.Fatal("Not an RSA private key")
	}

	// Call this if the key is going to be reused for multiple decryptions:
	// privateKey.Precompute()

	// Successfully loaded RSA private key
	fmt.Println("Successfully loaded RSA private key:")
	fmt.Printf("Modulus size: %d bits\n", privateKey.N.BitLen())

	// 2. Read ciphertext from a file (in this example, raw binary data)
	ciphertext, err := os.ReadFile("signed_message.bin")
	if err != nil {
		log.Fatalf("Failed to read ciphertext file: %v", err)
	}

	// 3. Decrypt using RSA-PKCS1v15
	plaintext, err := rsa.DecryptPKCS1v15(rand.Reader, privateKey, ciphertext)
	if err != nil {
		log.Fatalf("Failed to decrypt: %v", err)
	}

	fmt.Println("Decrypted message:")
	fmt.Println(string(plaintext))
}